Radiation curable cross linkable compositions

ABSTRACT

This invention realtes to a radiation curable cross linkable composition containing (a) from about 0.1 to about 5 wt. % of an initiator containing at least 25% cationic initiator, (b) from about 0 to about 60 wt. % of a polymerizable vinyl ether, expoxide, vinyloxy alkyl urethane or acrylate and (c) from about 35 to about 99.9 wt. % of a polyfunctional alkenyl ether having the formula 
     
         A[(CH.sub.2 O).sub.m (Z).sub.r CH═CHR].sub.n 
    
     wherein A is a carbon atom, --OCH═CHR or [C 1  to C 10  alkyl] 4-n  ; R is C 1  to C 6  alkyl; Z is C 2  to C 8  alkyleneoxy; r has a value of from 0 to 6; m has a value of from 0 to 1 and at least one of r and m has a positive value; n has a value of from 1 to 4, with the proviso that m is 0 and n is one when A is --OCH═CHR, n has a value of 2 or 3 when A is [C 1  to C 10  alkyl] 4-n  and n has a value of 4 when A is carbon. The invention also relates to the process of curing said composition and to a cured film on a substrate coated with the above composition.

This is a division of application Ser. No. 470,487, filed Jan. 26, 1990.

BACKGROUND OF THE INVENTION

Certain radiation curable coatings and films such as those formed fromthe acrylates, particularly trimethylol propane triacrylate,trimethacrylate, pentaerythritol triacrylate, and hexanediol diacylateor methacrylate, are in great demand because of their rapid curingproperties. However, these compounds are normally highly viscous liquidsor solids and thus are unsuitable as diluents for other polymericcomponents of a radiation curable formulation. Indeed, such compoundsthemselves require the incorporation of a diluent or solvent for uniformsubstrate coating, control of coating thickness and curing at lowtemperatures. Accordingly, low viscosity monofunctional diluents areusually included in their formulations. While these diluents arereactive, they materially reduce the cross-linked density of thefinished product and consequently lower abrasion resistance and abilityto withstand chemical attack.

Although solvents have been used to reduce viscosity, they aredetrimental in radiation curing due to their volatility which presentsproblems for uniform composition control unless their evaporation priorto radiant exposure is effected. Obviously, such procedure extendsprocessing time and may pose environmental drawbacks.

To some extent, the drawbacks of high viscosity monomers can be reducedby curing at elevated temperatures. However, this alternativesignificantly adds to the cost of the overall operation in theexpenditure of energy, temperature control and loss of more volatilecomponents in the composition or blistering of the coating resultingfrom entrained volatiles.

Since acrylate monomers are not conducive to cationically inducedradiation curing, they require free radical systems which are oxygeninhibited unless effected in an inert atmosphere, generally under ablanket of nitrogen. Although formulation with a photoinitiator whichundergoes bimolecular reaction with a hydrogen donor minimizes theinhibitory effect of air, this benefit is realized at the expense of agreatly reduced cure rate. Also, it is found that polymerization orcuring in free radical systems ceases almost immediately upon removalfrom the source of radiation; thus, the cured product often containssignificant amounts of unpolymerized components. Accordingly, it is anaim of research to develop a monomer having the beneficial properties ofacrylates but which is amenable to radiation curing at a rapid rate bycationically induced polymerization which is not oxygen inhibited andwhich permits continued polymerization after removal from the source ofradiation exposure.

The inherent deficiencies of the acrylate systems can be partiallyovercome by the use of epoxy resins. Epoxy resins can be polymerized bynormal radiation techniques using cationic photoinitiators such asiodonium, sulfonium and ferrocene salts of hexafluorophosphate,hexafluoroantimonate or hexafluoroarsonate to produce a tack free film.Although in such formulations tack free products are almost immediatelyobtained, polymerization of the mixture is incomplete. It is well knownthat the polymerization of epoxy resins is extremely slow and requiresas much as several days to achieve their ultimate physical properties.Thus, thermal post curing is often employed to increase the rate of orto complete the polymerization.

Certain allyl compounds also have been used as coatings; however thesemonomers and their oligomers are not readily curable by cationicradiation. Thermal curing is generally required to increase the rate ofpolymerization. While allyl ethers such as polyethylene glycols arecurable by UV light, they require a free radical initiated reactionwhich proceeds at a slow rate, generally over a period of from 2 to 10hours in order to reach completion.

Finally, it is noted that the unsubstituted acrylates are sensitizersand skin irritants as well as being carcinogenic, so that specializedsafety precautions must be taken to protect operators from exposure.Although alkoxylation has lessened irritancy of the acrylates, theircarcinogenic properties are not reduced.

Accordingly it is an object of the present invention to overcome theabove described deficiencies by an economical and commercially feasiblecomposition and curing process.

Another object of this invention is to utilize a multifunctionalcross-linking agent which is a liquid and which is more economicallyemployed in an efficient ether cross-linking process.

Another object is to provide a non-toxic cross linkable homopolymericcompound suitable as a film or a substrate coating which possesses goodadhesion, abrasion resistance and resistance to chemical attack.

Still another object is to provide a more economical process forcross-linking monomeric or polymeric vinyl or epoxy ethers which can beeffected in the presence of air.

Another object is to provide a monomer which is curable at a rapid rateby cationically induced radiation.

These and other objects will become apparent from the followingdescription and disclosure.

THE INVENTION

In accordance with this invention there is provided a radiation curable,cross linkable composition containing (a) from about 0.1 to about 5 wt.% of an initiator containing at least 25% of a cationic initiator, (b)from about 0 to about 60 wt. % of one or more polymerizable componentsof the group of a vinyl ether, epoxide, acrylate or a vinyloxy alkylurethane and (c) from about 35 to about 99.9% wt. % of an aliphaticpolyfunctional alkenyl ether having the formula

    A[(CH.sub.2 O).sub.m (Z).sub.r CH=CHR].sub.n

wherein A is a carbon atom, --OCH═CHR or [C₁ to C₁₀ alkyl]_(4-n) ; R isC₁ to C₆ alkyl; Z is C₂ to C₈ alkyleneoxy; r has a value of from 0 to 6;m has a value of from 0 to 1 and at least one of r and m has a positivevalue; n has a value of from 1 to 4, with the proviso that m is 0 and nis one when A is --OCH═CHR, n has a value of 2 or 3 when A is [C₁ to C₁₀alkyl]_(4-n) and n has a value of 4 when A is carbon.

Of the above polyfunctional alkenyl ether compounds, those wherein R ismethyl; A is --OCH═CH(lower alkyl), [lower alkyl]_(4-n) or carbon arepreferred. Also, when present alkenyl ether is asymetrical, the compoundmost preferably contains at least 35% of the cis isomer with respect tothe trans isomer.

The most preferred compositions are those containing between about 20%and about 50% of component (b) and between about 50% and about 80% ofcomponent (c) where R is methyl.

The present polyfunctional alkenyl, preferably propenyl, ether compoundsare homopolymerizable resins independently useful as protective coatingsand are also effective cross-linking agents for polymerizable vinylethers having at least 6 carbon atoms or epoxides such as the divinylethers of the bis(hydroxyethyl) ether of bisphenol A, the divinyl etherof triethylene glycol, the divinyl ether of dimethylolcyclohexane,vinyloxyalkyl urethanes, e.g. divinyloxybutyl urethane oligomers, thediglycidyl ether of bisphenol A and its oligomers, bisphenol A epoxyacrylate and its oligomers, 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate, the ethers disclosed in U.S.Pat. Nos. 4,388,450; 4,749,807; 4,775,732 and 4,751,271 andcorresponding alkoxylated compounds and similar comonomers in monomericor oligomeric form having a number average molecular weight up to about5,000 or mixtures of said comonomers and/or copolymers. Such monomericor polymeric vinyl ethers, epoxides, acrylates or urethanes can bereacted with the polyfunctional alkenyl ethers of this invention to forma cross-linked copolymeric product having a high cross-linked densityand extremely high resistance to abrasion and chemical attack.

As stated above, the present polyfunctional alkenyl ethers, particularlythe prop-1-enyl ethers, are homopolymerizable forming an exceedinglybranched structure. As such, these agents can be used as rigid coatingson substrates which require an exceptionally high strength, resistanceto abrasion and solvent attack. Substrates on which the copolymerized orhomopolymerized agent is suitably coated include metal, wood, ceramic,plastic, leather, paper, glass and the like. The present composition iscoated on the substrate by any convenient and conventional technique inthe desired thickness, usually in a thickness of between about 0.1 toabout 5 mils.

Instant alkenyl ethers having the structure C[CH₂ O(Z)_(r) CH═CHR]₄produce homopolymers and copolymers which are totally etheric incomposition and which have greatly increased surface substantivity andother advantages derived from their poly etheric nature, such as high UVresistance and the ability to form hydrogels on exposure to water.

As cross-linking agents, the alkenyl ethers of this invention can beadmixed with the above acrylate, urethane, epoxide or vinyl ethermonomers or their oligomeric counterparts to effect cross-linking in thepresence of a cationic initiator, such as a triphenyl sulfonium salt ofphosphorous hexafluoride, diphenyl iodonium salt, a mixture of aromaticcomplex salts of butyrolactone (FX-512, supplied by Minnesota Mining &Mfg. Co.), a phenyl onium salt or an aryl alkyl onium salt, etc. Theinitiators suitable to effect polymerization reactions of the presentinvention include the above named cationic initiators which can beemployed alone or in admixture with a free radical initiator to providea hybrid system. Suitable free radical initiators include1-hydrocyclohexyl phenyl ketone (e.g. IRGACURE 184),2-hydroxy-2-methyl-1-phenyl-1-propan-1-one (DAROCUR 1173),2,2-dichloro-1-(4-phenoxy- phenyl) ethanone (SANDORAY 1000) and thelike. Other free radical and cationic initiators which are suitablyemployed in this invention are those described by M.J.M. Abadie,Advantages and Development of Photochemical Initiators, in the EuropeanCoatings Journal 5/1988, pages 350-358. When initiator mixtures areemployed, the free radical component can comprise up to 75%, preferablybetween about 30 and about 70%, of the cationic initiator component. Aparticularly preferred initiator mixture includes between about 30 wt. %and about 40 wt. % of FX-512 and between about 60 and about 70% ofIRGACURE 184. The present initiator mixtures are recommended for blendsof (b) and (c) where component (b) contains an acrylate. The totalamount of initiator employed is generally between about 0.1 and about 5wt. % with respect to reactant or reactants.

In accordance with this invention, one or more of the present aliphaticalkenyl ethers can be employed or blended with one or more of thepolymerizable epoxides, vinyl ethers, acrylates or vinyloxy alkylurethanes, thus benefiting from the properties of each monomer in theblend. Further, it is found that blends of the present propenyl etherand the divinyl ether of dimethylol cyclohexane enhance solubilizationof the cationic initiator. Such blends may contain up to about 60%,preferably from about 20 to about 50% of component (b).

The propenyl ether of component (c) in the present composition, servesnot only as a reactant, but also as an essential diluent for the vinylether and/or epoxide which compounds are highly viscous and difficult toapply as coatings. Thus, the propenyl ether provides a coatablecomposition without the need for extraneous diluents which in many casescan cause blisters and non-uniformity in the coating product.

The compositions of the present invention are cured within a period ofup to one second by exposure to a source of radiation, e.g. UV light,electron beam, laser emissions, gamma rays etc. Radiation curing in thepresent cationic system takes place at a fast rate, e.g. from about 200to about 1,000 feet per minute of coated surface or free formed film,depending upon the intensity and type of radiation employed. UV lightradiation dosages at room temperature of from about 100 to about 1500milli J/cm² are effective and dosages of from about 200 to about 600milli J/cm² are preferred. Equivalent dosages for curing are employedwhen using alternative sources of radiation. For example, curing withelectron beam radiation can be carried out at between about 0.5 andabout 20 Mrads, preferably between about 1 and about 10 Mrads. Specifictechniques for radiation curing are well known, thus furtheramplification is not required.

Since the present propenyl ethers are normally liquid, they can bedirectly mixed with the polymerizable vinyl ether, epoxide or vinyloxyalkyl urethane monomer or oligomer without further conditioning;however, in certain cases where dilution is desired, as in cases wherehigher molecular weight alkenyl ethers of this invention are employed ascomponent (c) or where the blend provides a highly viscous mixtue, thealkenyl ether can be dissolved in an inert organic solvent such asmethyl ethyl ketone, toluene, a hydrocarbon, acetone, an ether or ahalogenated compound such as methylene chloride. However, dilution withthe above solvents should not exceed 50% when highly resistant coatingsare required.

Alternatively, the alkenyl ether monomer or oligomer, in the absence ofa comonomer can be applied directly to any of the above substrates andsubjected to radiation for curing under the above conditions to form amore highly cross-linked homopolymeric protective coating.

It should also be understood that the present compositions canoptionally contain minor amounts of conventional adjuvants such as asurfactant e.g. a fluorocarbon surfactant such as a mixture offluoroaliphatic polymeric esters (FC-430 supplied by Minnesota Mining &Mfg. Co.) or a silicane copolymer surfactant (DC-193 supplied by DowCorning Corp.) or others. It is also to be understood that the presentcompositions can be cured thermally or by radiation induced free radicalpolymerization; however, an advantage of this invention is the abilityto cure the compositions by cationically induced radiation which avoidsthe disadvantages discussed in the foregoing disclosure. It is to beunderstood however that concurrent free radical and cationic inducedpolymerization using a mixture of such photoinitiators achieves benefitsof this invention and is recommended where component (b) of thecomposition is an acrylate, e.g. bisphenol A epoxyacrylate.

Having generally described the invention, reference is now had theaccompanying examples which illustrate preferred embodiments but whichare not to be construed as limiting to the scope of the invention asmore broadly set forth above and in the appended claims.

EXAMPLE 1

Into an amber bottle, 50 wt. % of diprop-1-enyl ether of diethyleneglycol (70% cis, cis) and 50 wt. % of a diglycidyl ether of bisphenol Awere charged and mixed at 50° C. for 1 hour. To this mixture, 2parts/hundred parts of the triphenyl sulfonium salt ofhexafluorophosphate were added with agitation. The resulting lowviscosity liquid was directly coated on an aluminum panel in a thicknessof 0.15 mil. The coated substrate was then exposed for less than onesecond at room temperature to 400 milli J/cm² radiation from a mediumpressure mercury vapor lamp; after which the substrate having a highlycrosslinked strongly adhesive coating was removed by Cross Hatch Tapetes ASTM 3359. The coating is resistant to attack by methyl ethyl ketoneand is abrasion resistant.

EXAMPLE 2

A two mil thick layer of mixture of 98 wt % of the tetraprop-1-enylether of pentaerythritol and 2.0 wt. % of the triphenyl sulfonium slatof hexafluorophosphate is applied to a polyester substrate. The coatedlayer is then crosslinked by exposure for about one second at roomtemperature to electron beam radiation at a dosage of 3 Mrad. Theresulting highly crosslinked polymer exhibits strong adhesion, is highlyresistant to chemical attack and has superior abrasion resistantproperties.

EXAMPLE 3

A. Into an amber bottle, 50 grams of substantially pure (>95%) cis, cisdipropenyl ether of triethylene glycol, 50 grams of a diglycidyl etherof bisphenol A, 2 grams of the triphenyl sulfonium salt ofhexafluorophosphate and 1 gram fluorocarbon surfactant were charged andthoroughly mixed. The resulting liquid mixture was coated on an aluminumpanel with a #3 coating rod.

B. The above procedure was repeated except that a mixture of 48% cis,cis, 42% cis, trans mixture and 10% trans, trans was substituted for thecis, cis reactant in A.

C. The procedure in part A was repeated except that3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate wassubstituted for the dipropenyl ether.

Each of the samples A, B, and C were coated in a thickness of 0.11-0.15mil on an aluminum panel and then cured by exposure to UV light asdescribed in Example 1. The cured coatings were evaluated and theresults are reported in the following Table.

                                      TABLE                                       __________________________________________________________________________                    A           B         C                                                       NO BAKE*                                                                             BAKE**                                                                             NO BAKE                                                                             BAKE                                                                              NO BAKE                                                                             BAKE                              __________________________________________________________________________    Pencil Hardness (ASTM D3363)                                                                  F      2H   F     2H  H     2H                                % Adhesion (ASTM D3359)                                                                       --     100  --    100 --    100                               % Adhesion      --     100  --    100 --    100                               30 Min. Boiling H.sub.2 O                                                     Double MEK Rubs  1      2    1     2   1     2                                Reverse Impact (M-lbs)                                                                        --      65  --     60  40    40                               Mandrel Bonds (inch -                                                                         1/8    1/8  1/8   1/8 1/4   1/4                               ASTM D3111)                                                                   Coating Thickness                                                                             --     0.11 --    0.20                                                                              --    0.15                              Min Exposure for Tack-free                                                                    80     --   80    --  400   --                                coatings (m J/cm.sup.2)                                                       __________________________________________________________________________     *Immediately after exposure to 400 m J/cm.sup.2 UV.                           **Baked for 10 minutes at 170° C. after UV exposure.              

It will be appreciated from the above results that changing thedistribution from cis isomer to a cis/trans isomeric mixture did notmaterially affect the properties of the final coating. Example 3 alsodemonstrates the high cure speed of the di-propenyl ethers as comparedto the di-epoxy compound.

EXAMPLE 4

A. Into an amber bottle, 50 grams of substantially pure (>95%) cis,cis-dipropenyl ether of triethylene glycol, 50 grams of a bisphenol A epoxyacrylate oligomer, 1 gram silicone surfactant (DC-193), 1 gram cationicphotoinitiator (FX 512) and 1.5 gm free radical initiator (IRGACURE 184)were charged and mixed at 50° C. until homogeneous. The resulting liquidwas coated on polyester using a #6 coating rod (approx. 0.5 mil) andcured by an exposure for less than 1 second at room temperature to 400millijoules/cm² from a UV lamp.

B. The above procedure A was repeated except that the free radicalinitiator was omitted from the formulation.

C. The procedure in part A was repeated except that the cationicinitiator was omitted from the formulation.

The cured coatings were evaluated immediately after UV exposure and theresults are reported in the following Table.

                  TABLE                                                           ______________________________________                                        Formula       A            B        C                                         ______________________________________                                        Result        tack free    tack free                                                                              wet                                       Adhesion        100%       100%     none                                      Double MEK Rubs                                                                             >100         89       none                                      Pencil Hardness                                                                             F            F        none                                      ______________________________________                                    

This example illustrates the necessity of the cationic photoinitiatorand the superior solvent resistance obtained using a cationic and freeradical initiator.

EXAMPLE 5

A. Example 4A is repeated except that the 50 grams of cis,cis-dipropenyl ether of triethylene glycol is replaced with 50 grams of a 1to 1 wt. % blend of cis,cis-dipropenyl ether of triethylene glycol andthe divinyl ether of 1,4-cyclohexane dimethanol.

B. Example 4A is repeated except that the 50 grams of cis,cis-dipropenyl ether of triethylene glycol is replaced with 50 grams of thedivinyl ether of 1,4-cyclohexane dimethanol. The cationic initiatorfailed to dissolve in the absence of the propenyl ether and an incompatible mixture was formed.

The above cured coatings were compared with that of 4A and wereevaluated immediately after UV exposure. The results are as reported inthe following table.

                  TABLE                                                           ______________________________________                                        Formula     4 A        5 A       5 B                                          ______________________________________                                        Adhesion      100%       100%    incompatible                                 Double MEK Rubs                                                                           >100       >100      none                                         Pencil Hardness                                                                           F          2H        none                                         ______________________________________                                    

This example illustrates that coating hardness can be significantlyimproved by adding the divinyl ether of 1,4-cyclohexane dimethanol; andthat, the dipropenyl ether of triethylene glycol is needed to insurecompatibility.

EXAMPLE 6

Into an amber bottle, 50 grams of >95% cis, cisdipropenyl ether oftriethylene glycol, 50 grams of a divinyl ether of urethane oligomer(prepared as described in the Degree Thesis of Lennart Carlson, Dept. ofPolymer Technology, the Royal Institute of Technology, Stockholm,Sweden, 1987), 4 phr (parts/100 parts resin) cationic photoinitiator (FX512), and 1 phr fluorochemical surfactant (DC-193) were charged andmixed at 50° C. until homogeneous. The resulting liquid was coated on analuminum panel to a 0.25 mil thickness using a #3 coating bar and thencured as described in Example 4. A tack free coating with the followingproperties was produced

    ______________________________________                                        Pencil Hardness  3B                                                           Mandrel Bend     3/16 inch                                                    Double MEK rubs  5                                                            ______________________________________                                    

What is claimed is:
 1. The process which comprises forming a radiationcurable, cross linkable composition comprising (a) from about 0.1 toabout 5 wt. % of an initiator containing at least 25% cationicinitiator, (b) from about 0 to about 60 wt. % of a polymerizable vinylether, epoxy ether, epoxy acrylate and/or vinyloxy alkyl urethane and(c) from about 35 to about 99.9% wt. % of an aliphatic polyfunctionalalkenyl ether having the formula

    A[(CH.sub.2 O).sub.m (Z).sub.r CH═CHR]n

wherein A is a carbon atom, --OCH═CHR or [C₁ to C₁₀ alkyl]_(4-n) ; R isC₁ to C₆ alkyl; Z is C₂ to C₈ alkyleneoxy; r has a value of from 0 to 6;m has a value of from 0 to 1 and n has a value of from 1 to 4, with theproviso that m is 0 and n is one when A is --OCH═CHR, n has a value of 2or 3 when A is [C₁ to C₁₀ alkyl]_(4-n) and n has a value of 4 when A iscarbon, coating said composition on a substrate and curing saidcomposition on said substrate by exposure to an effective polymerizingdosage of radition for a period sufficient to produce a tack-free film.2. The process of claim 1 wherein component (a) of said composition is amixture of cationic and free radical initiators and between about 20 andabout 50% of component (b) is present in said composition.
 3. Theprocess of claim 2 wherein component (a) of said composition is betweenabout 30 and about 70 wt. % of a cationic initiator and between about 70and about 30 wt. % of a free radical initiator.
 4. The process of claim3 wherein component (b) of said composition contains an acrylatecomonomer which is present up to 60% of the composition.
 5. The processof claim 1 wherein the coated substrate is radiation cured in less thanone minute at room temperature.
 6. A substrate having a coating of thecomposition of claim
 1. 7. A substrate having a coating of thecomposition of claim 1 consisting essentially of the aliphaticpolyfunctional propenyl ether and a cationic initiator.
 8. A substratehaving a coating of the composition of claim 1 wherein said compositioncontains between about 20 and about 50% of component (b) and betweenabout 50 and about 80% of said alkenyl ether as a polyfunctionalpropenyl ether.
 9. A substrate having a coating of the composition ofclaim 1 wherein the aliphatic alkenyl ether is asymmetrical and is amixture containing at least about 35% cis isomer with respect to thetrans isomer.
 10. A substrate having coated on its surface across-linked film of the tetraprop-1-enyl ether of pentaerythritolhomopolymer.
 11. A substrate having coated on its surface a cross-linkedfilm of the dipropenyl ether of triethylene glycol and the diglycidylether of bisphenol A.
 12. A substrate having coated on its surface across-linked film of from about 20 to about 50% of diglycidyl ether ofbisphenol A and from about 50 to about 80% of the tetraprop-1-enyl etherof pentaerythritol copolymer.